An alternative and not inappropriate title would have been “An Outline of Social Psychology”; but two reasons prevented the adoption of this. First, my _Introduction to Social Psychology_ has become generally known by the abbreviated title _Social Psychology_. This was an unforeseen result and unfortunate designation; for, as I have explained in the Preface to the present volume, that other work was designed merely as a propaedeutic; it aimed merely at clearing the ground and laying the foundations for Social Psychology, while leaving the topic itself for subsequent treatment. Secondly, I conceive Group Psychology to be a part only, though a very large part, of the total field of Social Psychology; for, while the former has to deal only with the life of groups, the latter has also to describe and account for the influence of the group on the growth and activities of the individual. This is the most concrete part of psychology and naturally comes last in the order of development of the science; for, like other sciences, psychology began with the most abstract notions, the forms of activity of mind in general, and, by the aid of the abstract conceptions achieved by the earlier workers, progresses to the consideration of more concrete problems, the problems presented by actual living persons in all their inexhaustible richness and complexity. Until the later decades of the nineteenth century, psychology continued to concern itself almost exclusively with the mind of man conceived in an abstract fashion, not as the mind of any particular individual, but as the mind of a representative individual considered in abstraction from his social settings as something given to our contemplation fully formed and complete. Two important changes of modern thought have shown the necessity of a more concrete treatment of psychological problems. The first has been the coming into prominence of the problems of genesis which, although not originated by Darwin, received so great an impetus from his work. The second has been the increasing realisation of the need for a more synthetic treatment of all fields of science, the realisation that analysis alone carries us ever farther away from concrete problems and leads only to a system of abstract conceptions which are very remote from reality, however useful they may prove in the physical sciences. The biological and the human sciences especially have been profoundly affected by these two changes of modern thought. As Theodore Merz has so well shown in the fourth volume of his monumental work[1], the need has been increasingly felt of the _vue d’ensemble_, of the synthetic mode of regarding organisms, men, and institutions, not as single things, self-contained and complete in themselves, but as merely nodes or meeting points of all the forces of the world acting and reacting in unlimited time and space.